The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to acoustic vibrator type seismic energy sources used for marine seismic surveying.
Seismic sources are used in geophysical surveying both on land and in water-covered areas of the earth. Signals generated by seismic sources travel downwardly into the earth, are reflected from interfaces in the subsurface, and are detected by signal detectors, typically hydrophones or geophones, on or near the earth's surface or water surface.
Most of the seismic sources used in marine surveying operations are of the impulsive type, in which efforts are made to generate as much acoustic energy as possible during as short a time span as possible. Examples of such sources include air guns, water guns, and arrays of such guns. The frequency content of such sources is controllable only to a small degree, principally by adding additional guns with different charge chamber sizes to a gun array. Different source arrays are selected for the generation of different frequency ranges for different surveying needs. Impulsive sources generally have a limited acoustic output in the very low frequency band from 1-10 Hz, and especially below 5 Hz.
Another type of seismic source is an acoustic vibrator. Acoustic vibrator type sources known in the art include hydraulically powered sources, conventional flextensional shell sources, and sources employing piezoelectric or magnetostrictive material. Acoustic vibrators tend to offer better frequency control than impulsive sources. Similar to impulsive sources, acoustic vibrators generally have a very limited acoustic energy output below 10 Hz.
Typical flextensional shell sources are based on the principle of changes in volume in a vibrating, generally elliptic shell. When the longer, major axis of an ellipse is set into vibration by a driving force (e.g., an electro-dynamic driver), the length of the shorter, minor axis will also vibrate, but with a much larger amplitude. However, for very low frequencies it can be rather problematic to generate enough amplitude by standard flextensional shell sources (e.g., using piezoceramic or Terfenol-D type of driver attached to the end of the major axis in the ellipsoid). For example, since the force generated drops-off rapidly with distance between the magnets, many conventional electro-magnetic drivers may be unable to generate sufficient force for large-amplitude applications. Some flextensional shell sources use additional mechanisms to further enhance the driving force applied to the major axis of the shell. Examples of such may be found in U.S. Pat. Nos. 5,959,939, 6,076,629 and 7,551,518, each issued to Tenghamn, and each herein incorporated by reference.
It is known in the art that, as acoustic waves travel through water and through subsurface geological structures, higher frequency acoustic waves are attenuated more rapidly than lower frequency waves. Consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves.
A generally elliptical flextensional shell source may be designed with a low fundamental resonance frequency so that the shell's dimensions are small compared to the wavelength in water, thereby allowing the flextensional shell to radiate sound omnidirectionally. However, due to the relatively small size of the flextensional shell (compared to the wavelength in water), the acoustic load is low at low frequencies and strongly reactive, typically requiring a large velocity amplitude of the radiating surface. Hence the allowable maximum stress in the shell is a limitation in the power output of typical elliptical flextensional shells with low resonance frequencies.
There has been a long standing need in the seismic sector of the oil and gas industry for powerful, low frequency marine sound sources operating in the frequency band 1-10 Hz.